Small RNAs are known to perform diverse cellular functions, including post-transcriptional gene expression regulation. The first two such RNA genes, Lin-4 and Let-7, were identified by genetic analysis of Caenorhabditis Elegans (Elegans) developmental timing, and were termed short temporal RNA (stRNA) (Wightman, B., Ha, I., Ruvkun, G., Cell 75, 855 (1993); Erdmann, V. A. et al., Nucleic Acids Res. 29, 189 (2001); Lee, R. C., Feinbaum, R. L., Ambros, V., Cell 75, 843 (1993); Reinhart, B. et al., Nature 403, 901 (2000)).
Lin-4 and Let-7 each transcribe a ˜22 nucleotide (nt) RNA, which acts a post transcriptional repressor of target mRNAs, by binding to elements in the 3′-untranslated region (UTR) of these target mRNAs, which are complimentary to the 22 nt sequence of Lin-4 and Let-7 respectively. While Lin-4 and Let-7 are expressed at different developmental stage, first larval stage and fourth larval stage respectively, both specify the temporal progression of cell fates, by triggering post-transcriptional control over other genes (Wightman, B., Ha, I., Ruvkun, G., Cell 75, 855 (1993); Slack et al., Mol. Cell 5, 659 (2000)). Let-7 as well as its temporal regulation have been demonstrated to be conserved in all major groups of bilaterally symmetrical animals, from nematodes, through flies to humans (Pasquinelli, A., et al. Nature 408, 86 (2000)).
The initial transcription product of Lin-4 and Let-7 is a ˜60-80 nt RNA, the nucleotide sequence of the first half of which is partially complimentary to that of its second half, therefore allowing this RNA to fold onto itself, forming a ‘hairpin structure’. The final gene product is a ˜22 nt RNA, which is ‘diced’ from the above mentioned ‘hairpin structure’, by an enzyme called Dicer, which also apparently also mediates the complimentary binding of this ˜22 nt segment to a binding site in the 3′ UTR of its target gene.
Recent studies have uncovered 93 new genes in this class, now referred to as micro RNA or miRNA genes, in genomes of Elegans, Drosophilea, and Human (Lagos-Quintana, M., Rauhut, R., Lendeckel, W., Tuschl, T., Science 294, 853 (2001); Lau, N. C., Lim, L. P., Weinstein, E. G., Bartel, D. P., Science 294, 858 (2001); Lee, R. C., Ambros, V., Science 294, 862 (2001). Like the well studied Lin-4 and Let-7, all newly found MIR genes produce a ˜60-80 nt RNA having a nucleotide sequence capable of forming a ‘hairpin structure’. Expressions of the precursor ˜60-80 nt RNA and of the resulting diced ˜22 nt RNA of most of these newly discovered MIR genes have been detected.
Based on the striking homology of the newly discovered MIR genes to their well-studied predecessors Lin-4 and Let-7, the new MIR genes are believed to have a similar basic function as that of Lin-4 and Let-7: modulation of target genes by complimentary binding to the UTR of these target genes, with special emphasis on modulation of developmental control processes. This is despite the fact that the above mentioned recent studies did not find target genes to which the newly discovered MIR genes complementarily bind. While existing evidence suggests that the number of regulatory RNA genes “may turn out to be very large, numbering in the hundreds or even thousands in each genome”, detecting such genes is challenging (Ruvkun G., ‘Perspective: Glimpses of a tiny RNA world’, Science 294, 779 (2001)).
The ability to detect novel RNA genes is limited by the methodologies used to detect such genes. All RNA genes identified so far either present a visibly discernable whole body phenotype, as do Lin-4 and Let-7 (Wightman et. al., Cell 75, 855 (1993); Reinhart et al., Nature 403, 901 (2000)), or produce significant enough quantities of RNA so as to be detected by the standard biochemical genomic techniques, as do the 93 recently detected miRNA genes. Since a limited number clones were sequenced by the researchers discovering these genes, 300 by Bartel and 100 by Tuschl (Bartel et. al., Science 294, 858 (2001); Tuschl et. al., Science 294, 853 (2001)), the RNA genes found can not be much rarer than 1% of all RNA genes. The recently detected miRNA genes therefore represent the more prevalent among the miRNA gene family.
Current methodology has therefore been unable to detect RNA genes which either do not present a visually discernable whole body phenotype, or are rare (e.g. rarer than 0.1% of all RNA genes), and therefore do not produce significant enough quantities of RNA so as to be detected by standard biochemical technique.